1. Field of the Invention
The present invention generally relates to power converters and, more particularly, to resonant power converters including synchronous rectifiers having driving arrangements to control the switches thereof.
2. Description of the Prior Art
Many familiar electronic device are designed to operate with a power supply providing a substantially constant voltage while power may ultimately be derived from alternating current distribution systems or batteries at a different voltage. Therefore, power conversion arrangements are generally required as power supplies for such devices. Since power supply circuitry to provide a required voltage does not otherwise contribute to the function of the device to which power is supplied, there is a strong incentive to minimize the size and cost of such power supplies.
High power density (e.g. low-volume devices capable of supplying a large amount of power) power converters must be of very high efficiency since any losses in the power converter must be dissipated from the low volume and surface of the power converter, itself. Such power converters must also be of high performance to accommodate large changes in load current such as between idle and active power states of a digital processor with acceptable transient response. Applications such as on-board distributed power systems have increased interest in development of power converter densities which exceed 50 Watts per cubic inch and have shifted circuit design emphasis to high frequency (e.g. above 1 MHZ) resonant topologies for practical embodiments of which, efficiencies approaching 90% are required.
While diodes have been used in the past in power converter designs, power MOSFETs have much lower forward voltage drop than, for example, Schottky diodes and thus draw less power. Thus, synchronous rectification, where a transistor is controlled to conduct at approximately the periods that a diode would conduct, has become a design of choice for such applications. However, for a synchronous rectifier (SR) to exhibit an advantage over a Schottky diode, very precise control over the switching control waveform (and thus switching times) is required in order to obtain advantageously reduced switching losses.
Unlike pulse width modulated switching converters which have also been used in such applications, the turn-on times of switches on the primary side of the power converter circuit and the turn-on times of switches in the SR on the secondary side of the circuit are not exactly in phase for resonant converters and thus cannot use the same driving signal for control of conduction times. Otherwise, the SR would conduct recirculating energy, namely a reverse current from the load to the source; thus causing much increased RMS currents and causing efficiency to deteriorate dramatically. So-called reverse recovery losses are due to a component of such reverse current and result from non-ideal diode behavior of switching elements, whether constituted by diodes or the body diode of a transistor such as a MOSFET. Therefore, some different driving arrangement for the SR is required in some types of power converters.
For some types of resonant converters such as parallel resonant converters and series-parallel resonant converters, the transformer winding voltage may be used to drive the SR. However, for other types of synchronous rectifiers such as those in series resonant converters and LLC resonant converters, exemplary circuits of which are illustrated in FIGS. 1 and 2, respectively, the polarity of the voltage on the secondary winding can change only after the SR is turned off. In other words, when the SR is on, the voltage on the secondary winding of a transformer or autotransformer is clamped to the output voltage and thus cannot change polarity before the SR is turned off and, as a result, the voltage on the secondary winding cannot be used to control the SR (e.g. the SR must be controlled prior to the change in polarity on the transformer secondary).
To solve this problem, it has been proposed to utilize the leakage inductance of the transformer to reshape the terminal voltage of the transformer secondary in order to derive a signal to drive the SR. However, due to additional voltage stress caused by such an arrangement, such a solution is only suitable for power converter structures having a very low voltage output, usually limited to a very few volts. Further, such an arrangement is strongly dependent on di/dt effects.
It has also been proposed to use transformer secondary currents for SR control drive signals: when the body diode of the SR conducts, the SR is turned on and when the current becomes zero or becomes negative, the SR is turned off. However, this proposed SR control scheme suffers from the obvious drawbacks that a current transformer is awkward to incorporate into a power converter (due to its size) and leads to increased size and cost while its insertion inductance greatly compromises high frequency performance such as high voltage stress and higher switching losses.